This has to do with issue of hooking up a second monitor to your system but no picture showing on the screen on one of your monitors.
I've had this problem when setting up dual monitors before. I unplugged my second monitor for reasons and when I plugged it back on the screen remained black.
The solution is simple, right click on your desktop, and click on Screen resolution.
In the rectangular box that shows your monitor setup, on the right hand side is a button labled Detect. Just click on it.
Usually this clears up the problem. Another thing, at one time my monitor started displaying an odd resolution. Everything looked extra big on the screen. The resolutions where all set as they where and adjusting them had no effect.
Again clicking on this Detect button solved that problem as well.
There is also a Identify button there. This just displays a big white number on your monitors. Number 1 for the first, number 2 for the second and so on.